bpf: Selftests, add printk to test_sk_lookup_kern to encode null ptr check

Adding a printk to test_sk_lookup_kern created the reported failure
where a pointer type is checked twice for NULL. Lets add it to the
progs test test_sk_lookup_kern.c so we test the case from C all the
way into the verifier.

We already have printk's in selftests so seems OK to add another one.
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159009170603.6313.1715279795045285176.stgit@john-Precision-5820-Tower

tools/testing/selftests/bpf/progs/test_sk_lookup_kern.c

@@ -73,6 +73,7 @@ int bpf_sk_lookup_test0(struct __sk_buff *skb)
 
 	tuple_len = ipv4 ? sizeof(tuple->ipv4) : sizeof(tuple->ipv6);
 	sk = bpf_sk_lookup_tcp(skb, tuple, tuple_len, BPF_F_CURRENT_NETNS, 0);
+	bpf_printk("sk=%d\n", sk ? 1 : 0);
 	if (sk)
 		bpf_sk_release(sk);
 	return sk ? TC_ACT_OK : TC_ACT_UNSPEC;